1. Field of Invention
The present invention relates to a screw conveyor for conveying powder, and more particularly to a screw conveyor for use in a mechanism of conveying/recovering developer which is employed in an image forming apparatus.
2. Conventional Art
A conventional screw conveyor for use in a developer conveying/recovering mechanism has a structure such that an entire conveying blade 102 is completely fixed to a rotating shaft 101 as shown in FIG. 6(a).
When such a screw conveyor is used to convey developer, the conveying performance thereof varies depending on the fluidity of the developer. Developer itself usually has a high level of fluidity. However, if exposed to environmental changes, particularly a high humidity condition, each developer absorbs moisture in the air to expand, resulting in a decrease in the fluidity thereof. This is clearly seen from developer in a storage container with a history of exposure to high temperature and high humidity. Such developer is supplied as lumps of powder but not as individual powder particles. The fluidity of such lumps of developer is low.
Image information which is developed on an electrostatic latent image carrier (using developer) is transferred onto a paper sheet. In this case, there is residual developer (waste developer) which is not transferred onto the electrostatic latent image carrier. A cleaning unit is provided so as to remove the residual developer from the electrostatic latent image carrier. However, the cleaning unit removes from the electrostatic latent image carrier not only the residual developer but also dust adhering to a surface of a paper sheet conveyed to the transfer step, short fiber constituting paper (mainly, cellulose fiber), filler (SiO2, etc.) contained in paper, and the like. If these impurities are thus mixed into the waste developer, the dust and the short fiber entangle, resulting in a reduction in the fluidity of the waste developer.
When conveying the waste developer whose fluidity is thus reduced, the conveying blade 102 supported by the rotating shaft 101 is affected by greater rotational load as the weight of the conveyed waste developer is increased. During the rotation, heat is generated between the conveying blade 102 and a side wall surface or the like of a developer holding unit 105. The heat phenomenon softens the waste developer which in turn adheres to the conveying blade 102, the side wall surface of the developer holding unit 105 or the like, leading to a deterioration in conveying performance. If such a condition is continued, the waste developer is accumulated on and around the screw conveyor, resulting in a “blocking phenomenon” or clogging of the screw conveyor.
A description will be given of how such a blocking phenomenon occurs with reference to FIGS. 6(b) to 6(e) which show schematic cross-sectional views of a conventional screw conveyor.
FIG. 6(b) shows an initial state of the screw conveyor in a conventional cleaning unit (immediately after the apparatus is installed or maintenance is ended).
Waste developer (shaded in the figure) 103a is conveyed toward a waste developer container not shown (in a direction indicated by arrow X3 in the figure) by rotation of the rotating shaft 101 of the screw conveyor and inclination of the conveying blade 102 while the waste developer 103a is accumulated on a bottom 105a of the developer holding unit 105. If such a condition is continued (as the number of sheets printed by the apparatus is increased), the amount of the waste developer 103a is gradually increased in the developer holding unit 105 as shown in FIG. 6(c). Finally, as shown in FIG. 6(d), the developer holding unit 105 is filled with the waste developer 103a. 
Thus, as the amount of the waste developer 103a is increased in the developer holding unit 105, impurities (dust, short fiber, filler, etc.) other than the developer reduce the fluidity of the waste developer 103a. The waste developer 103a having a low fluidity becomes rotational load to the screw conveyor, and adheres to the rotating shaft 101 and the conveying blade 102 to further increase the rotational load.
As the rotational load is thus increased, the poor fluidity prevents the waste developer 103a from being conveyed in the direction of arrow X3, so that the waste developer 103a is rotated at a fixed position. As a result, the waste developer 103a is rubbed against the side wall or the like of the developer holding unit 105, leading to an increase in temperature. The increased temperature causes aggregation of the waste developer 103a, resulting in the blocking phenomenon. FIG. 6(e) shows the blocking phenomenon which thus occurs in the developer holding unit 105. When the rotational torque of the rotating screw conveyor is large, the waste developer 103a is not present on the conveying blade 102 and lumps of the waste developer 103a are accumulated in the vicinity of the conveying blade 102 as shown in FIG. 6(e). On the other hand, when the rotational torque of the screw conveyor is small, the screw conveyor is not rotated, so that combined gears coupled with a drive source are damaged.
The phenomenon occurs significantly at high temperature and high humidity. Specifically, the waste developer swells due to moisture in the air, thereby making the problem worse. An actual experiment performed by the present inventors observed a blocking phenomenon (condition of FIG. 6(e)) caused by waste developer, in which a screw conveyor could not be seen from the top at the time when 15,000 sheets had been printed after maintenance.
In order to prevent such a blocking phenomenon and a clogging phenomenon of a screw conveyor, a technique has been proposed in which the center axis of a conveying blade is shifted from the center axis of the rotating shaft of a screw conveyor so that the conveying blade is eccentrically rotated during screw rotation for conveying developer (see, for example, JP 2002-108160A).
A mechanism of a screw conveyor which conveys powder, such as developer or the like, is that powder built up on a wall or the like of a developer holding unit is lifted up and moved forward by rotation of an inclined conveying blade of the screw.
However, the technique of JP 2002-108160A has a new problem with the eccentricity of the screw conveyor. Specifically, the screw is departed from the center due to rotation. Therefore, the force of lifting the powder (developer) buildup on the wall or the like of the developer holding unit is reduced (this is because the eccentric situation causes the outer edge of the blade to approach or leave the wall surface), leading to a reduction in conveying performance.